In a manufacture of an integrated circuit of a semiconductor product, an object to be processed such as a semiconductor wafer is generally subjected to various processes such as a film-deposition process, modification process, oxidation and diffusion process, etching process, and so on. In recent years, a semiconductor circuit has been required to be further miniaturized while having an increased density and a thinner film thickness. Also, an improvement in yield of products is desired. Thus, in the etching process, there is an ongoing need to precisely form an etching shape as specified in the design and simultaneously to maintain a high selectivity of a layer to be etched relative to a base layer.
In the etching process, an etching gas is generally selected considering a selectivity between a layer to be etched and a base layer positioned below the layer to be etched to serve as an etching stopper layer. There has been a tendency to utilize a plasma in the etching process in order that an object to be processed can be etched at a lower temperature and that an etching efficiency can be enhanced (see, JP11-111689A and JP2004-39935A).
Given herein as an example to describe a conventional etching process is a case where a pattern is etched in a tungsten-containing film formed on a polysilicon layer so as to make a gate electrode.
FIG. 8 is an enlarged sectional view of a part of an object to be processed. As shown in FIG. 8, the object to be processed includes a semiconductor wafer W such as a silicon substrate. An SiO2 film 2 to become a gate oxide film, and a base layer 4 formed of a silicon-containing material, e.g., a polysilicon layer, are formed on a surface of the semiconductor wafer W. A layer to be etched 6 made of a tungsten-containing material is formed on the base layer 4. In this example, the layer to be etched 6 has a laminated structure including a tungsten nitride (WN) film 6A as a lower layer and a tungsten (W) film 6B as an upper layer. A patterned mask 8 for forming a gate electrode is formed on an upper surface of the layer to be etched 6. An SiN film, an SiO2 film, or a photoresist film may be used as the mask 8, for example.
The semiconductor wafer W having the above laminated structure has been conventionally etched by using an etching gas such as a Cl2 gas, NF3 gas, and SF6 gas, and using an additive gas such an O2 gas and the like for enhancing the selectivity relative to the base layer. These gases are activated by a plasma for the etching process. An inert gas such as an Ar gas is sometimes supplied according to need. Alternatively, an N2 gas may be used in place of the O2 gas.
The above-described semiconductor wafer W is etched according to the following manner. The etching gas is ionized by a plasma and activated to generate active species, and the active species attack the layer to be etched 6. At this time, the material of the layer to be etched 6, the active species, and the additive gas are reacted with each other to generate a reaction product. The reaction product is vaporized and discharged. As a result, all the parts of the layer to be etched 6 excluding parts protected by the mask 8 are scraped. The etching operation stops at the base layer 4 serving as a stopper layer.
Various substances are generated as reaction products during the etching process. A reaction product such as WF and WOCl4 causes no problem because of a relatively high volatility thereof. However, a reaction product such as WO3 and WClxOy (x and y respectively represent positive numbers), which is produced when a Cl-group gas is used as the etching gas, is disadvantageous in that such reduction product is prone to be deposited because of a relatively low volatility thereof.
As described above, when a flowrate of the O2 gas is increased, the selectivity relative to the base layer 4 can be elevated. However, with the increased flowrate of the O2 gas, the reaction product such as WO3 and WClxOy, which is difficult to be evaporated, may adhere to an etched part to degrade an etching shape. On the other hand, when an F-group gas, which is highly reactive, is used as the etching gas, the layer to be etched 6 may be excessively scraped to produce an undercut resulting also in a degraded etching shape.
FIGS. 9A to 9C are pictures taken by an electron microscope and schematic views thereof respectively showing examples of etching shapes formed by the conventional etching process. FIG. 9A is the schematic view showing an appropriate etching shape obtained by the appropriate etching process. FIG. 9B shows an etching shape obtained by using an N2-added Cl2 gas as the etching gas. In this etching process, a nonvolatile deposition 10 such as WO3 and WClxOy was deposited on a periphery of the etching shape. FIG. 9C shows an etching shape obtained by using an O2-added Cl2 gas as the etching gas. In this etching process, a periphery of the etching shape was excessively, arcuately scraped to produce an undercut part 12. That is to say, as shown in FIG. 9B and FIG. 9C, the conventional etching process may invite a degraded etching shape. On the other hand, the use of only the Cl2 gas as the etching gas extremely decrease the selectivity, so that the polysilicon layer as a base layer may be undesirably scraped.
This degradation in etching shape could be negligible heretofore owing to a not so strict dimension design rule regarding a line width, for example. However, with the stricter dimension design rule to achieve a further miniaturization, the degradation in etching shape is now to be solved.